Anodic coating of zinc base metals



Patented Apr. 10, 1934 f comrsosrarss PATENT OFFICE 1,953,998 ANODICCOATIING or ZINC BASE METALS Edward Cushman Truesdale and Ernest John Wilhelm, Palmerton, Pa., assignors to The New Jersey Zinc Company, New York, N. Y., a corporation of New Jersey No Drawing. Application November 14, 1932 Serial No. 642,609 I 14 Claims.

This invention relates to'the coating of .zinc

base metals by anodic action, and has for its.

object the provision of an improved method of producing useful anodic coatings on such metals,

as well as the provision, as new articles of comm the production of anodiccoatings on zinc and merce, of zinc base metals so coated. By zinc base metal"- we mean metals and alloys composed principally of zinc.

The invention is particularly concerned with zinc base alloys for the purpose of protection, ornamentation, or of serving as -a base for the ap-- plication of paints, lacquers and the like so as to secure good adhesion of the latter. Ordinary commercial zinc (in particular, rolled or strip zinc), zinc basealloys such as die-casting alloys containing over 90% zinc (for example, the alloys of United States Patents Nos. 1,596,761;

1,663,215, 1,779,525, 1,852,434, 1,852,441 and 1,852,442), and wrought zinc base alloys (for example, the alloys of United'States Patents Nos.

1,716,599, 1,832,653, 1,832,733 and 1,835,450) may be advantageously coated'by the method of the invention. Numerous other .zinc base alloys, particularly those containing small amounts of copper and/or aluminum, are likewise amenable to treatment in accordance with theinvention.

In our. copending application for Letters Patent of the United States, Serial No. 642,608, filed November 14,1932, we have disclosed the production of anodic coatingsion zinc from stronglyv basic electrolytes, such as 0.2N to LON sodium hydroxide solutions. .Anodic coatings produced with such strongly alkaline electrolytes are dark in color and 'are'comp'osed for themost part of zinc oxide. In thev course of our investigations, we have discovered that'a moredilute alkaline electrolyte produces a distinctly different type of coating. ;Thus, we have found that as the con-- centration-of sodium hydroxide is reduced from the 0.2N solution,'the color of the anodic coating becomes lighter, changing from dark gray through light gray to whitewith an 0.003N to 0.001N solution. We have found that in the case of concentrations of'NaQI-I not-greatly below 0.2N, the color of the coating is rendered lighter by increasing the eurrent'density. Based on these discoveries, our present invention involves, in its broad aspect, producing an anodic coating on a zinc base metal by subjecting the metal as anode to electrolytic treatment in an alkaline electro lyte having a hydroxyl ionconcentration less than the equivalent of a- 0.2N sodium hydroxide solution. --Themaximum pH value of such a dilute alkaline electrolyte is about13.3;. commonly 'a pH substantially below 133 is preferable. It

is characteristic of the invention that the hy- "droxyl ion concentration of the electrolyte is such as to produce on the zinc anode a coherent, light-colored coating composed for the most part of zinc oxide. Y In carrying out the invention, the articles of zinc orfzinc base alloys to be coated are subjected to an electrolytic treatment as anode while immersed in a relatively dilute alkaline electrolyte. When so treated at appropriate 'voltages, current densities and temperatures, the zinc articles become coated with a firm adherent layer of zinc oxide and/or hydrated zinc. oxide formed byv reaction of zinc and hydroxyl ions. The

coatings are light gray to white in color, depend ing upon the hydroxyl ion concentration and other operating conditions, in particular, the coatings are rendered lighter in color by increas.-- ing the current density. If desired, the coatings.

may be dyedwith suitable dyes to produce a considerable range'of. color effects.

The coatings ordinarily produced on zinc by, corrosion in solution (e. g. alkaline solutions) are in general relatively loose and poorly adherent. In contrast with this, the'anodic' coatings produced in accordance with the present invention are thi'n, dense and firmly adherent. These anodic coatings usually reproduce in minute detail the surface markingsoriginally present'on the treated metal objects.

This is probably due to the fact that the coating is formed primarily by appearance of the coating may be improved by bufling thezinc article before anodic treatment. This bufilng procedure may advantageously be of the same character as used on zinc prior to electroplating Other preliminary treatment, such as sand-blasting or-etching which, like buffing, removes a surface film especially likely to be present on cast zinc surfaces, may be employed to facilitate the subsequent anodic treatment. Such preliminary surface treatment of the zinc after one-half to three minutes 'of cleaning. After cleaning, the zinc article is rinsed in hot and then in cold water to removethe residual cleaning solution, following which the zinc article is ready for anodic treatment.

A satisfactory electrolyte for the practice of the invention is a 0.025N to 0.003N solution of sodium hydroxide. The electrolytic treatment is conducted at room temperature. The operation may be started at about or 'volts, but the hard,

' adherent, anodic films establish a high degree of polarization or offer great resistance so that .it may be desirableto raise the voltage to from to 90 volts. The hardness of the anodic films may bedue to the high polarization. The anodic coatings produced in such dilute alkaline .electrolytes are light gray to white and frequently show a red and green iridescence.

The following table gives specific data on the production of satisfactory coatings on a zinc base die-casting alloy containing approximately 4% aluminum and 0.1% magnesium, the balance be.- ing zinc metal-of high purity (99.99+% Zn). In carrying outthese operations, the-voltage was started at 10 or .12 volts and then gradually increased during a period of 5 minutes to the maximum shown in the table, as the current density was decreased by the polarization or resistance resulting from the coating. This method of op-- .eration is preferable, since a less uniform coating is obtained if the maximum voltage is applied at the start. These coatings consist essentiallyv of zinc oxide and being light in color .may be dyed with suitable dyes such .as alizar-ine dyes:

Concentration of M Minimum cur- C I f of NaOH in the mmum rent density 0 or o electrolyte voltage (at anode) mung A/Square ne 30 Gray. s a sagray. 0o 2.5-5 Wino.

The operating life of these electrohrtes, especially the'more dilute ones, is relatively short due to the accumulation of zinc in the solution and to the absorption of carbon dioxide from the air.

; Withcontinued use these electrolytes gives a loose, fiocculent' anodic deposit instead of the thin, adherent type obtained under the operating conditions characteristic of the invention.

The invention may be practiced with electrolytes containing from 0.2N to 0.5N amomnium hydroxide (NH4OH) solutions, .at room temperature and with a voltage of 6 to 12 at the source. Below a concentration of about 0.2N (NH4OH) the conductivity of the solution is so small as to preclude the formation of useful films in a reasonable time, while at concentrations 'above-0.5N (NH-40H) the chief action at the anode is etching of the zinc, so that the film formed at the anode is imperfect. The coatings obtained with an electrolyte of from 0.2 to 0.5N (NHiOH), un-

der the aforementioned conditions, and with ourrent densities up to the maximum obtainable, are

dark gray, becoming lighter at the higher voltages (12).

If ammonium hydroxide is added to dilute sodium' hydroxide solutions, the anodic films produced may in some cases be darkened to a certain extent by the presence of the ammonium hydroxide, while at the same time the sodium hydroxide imparts a much higher conductivity to the solution than that secured by the use of ammonium hydroxide-alone. The color of the film (light or dark) is dependent on the current density (and thus on the voltage applied) as well as on the relative proportions of NaOH and NHiOH.

Solutions of sodium carbonate containing sufficient'NazCOa to form zinc oxide coatings. say 100 grams or more of NMCO: per liter, may be used in the practice of the invention particularly when a solution life greater than that afforded bydilute hydroxide solution is desired. The pH value of a solution containing 200 grams per liter of sodium carbonate corresponds roughly to that of an 0.01N sodium hydroxide solution, and the anodic coatings produced with the two-solutions are similar in character. 1 With electrolytes containing from 100 to 200 grams. per liter of NaaCOa. the anodic films act to impede the current fiow throughpolarization or resistance, sothat voltages as high as to can be applied to the anode without breaking down the coating. The anodic coating consists essentially of zinc oxide, is very thin. uniform in appearance and light gray 110 to white; or colored by interference effects.

The anodic coatings of the invention not only protect the zinc or zinc alloy from corrosion but are also useful for decoration. These coatings may also be used as a base to increase the adherence of paint, enamel, lacquer etc. The lightcolored anodic coatings produced in accordance with the invention may be given various colors by dyeing. The dyeing may take place during the electrolytic treatment by solution of the dye in the electrolyte, or dyeing may be, carried out by immersion of the coated article in an appropriate solution of the dye. t

The operating conditions (including hydroxyl ion concentration of electrolyte) specified in con- 12 nection with the foregoing examples-of the practice of the invention may be varied without departing from the spirit-and scope of the invention. It is characteristic of the invention that the anodic coating consists essentially of zinc oxide and no is light in color. The color of the coating depends largely upon the concentration of hydroxyl ions in the electrolyte, becoming lighter as the concentration is decreased.

'The cathode used in, the anodic treatment of 185 the invention may-be of nickel, iron or other metal insoluble in the electrolyte.

We claim: 1. As a new article, of commerce, a zinc base metal; having a thin, light-colored, dense and firmly adherent anodic coating composed essentially of zinc oxide.

2. The method of producing a'coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkaline electrolyte having a pH less than 13.3.

3. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkaline electrolyte having a hydroxyl ion concentration 15 base metal which comprises subjecting the metal 'less than the equivalent of a 0.2N

sodium hydroxide solution. v

4. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkaline electrolyte having ahydroxyl ion concentration equivalent to that of a 0.1N to 0.001N hydroxide solution.

5. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in a dilute alkaline electrolyte having a hydroxyl ion concentration adapted to produce on said zinc base metal a coherent, light-colored coating consisting essentially of zinc oxide.

6. The method of producing a coating on a zinc sodiumas anode to electrolytic treatment in an alkaline electrolyte having a concentration of free alkali less than the equivalent of a 0.2N sodium ydroxide solution. j Y

of sodium hydroxide, and increasing the voltage as the current density decreases.

10. The method of producing a coating on, a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an 0.025N to 0.001N solution of sodium hydroxide, and increasing the initial voltage of about 10 volts to 30 or more volts as the current density decreases.

11. The methodof producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an electrolyte containing from 0.2N to 0.5M ammonium hydroxide.

12. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an electrolyte containing a sodium carbonate in amount adapted to form a zinc oxide coating;

13. The method of producing a coating on a .zinc base metal which comprises subjecting themetal as anode to electrolytic treatment in a dilute alkaline electrolyte containing sodium hydroxide and ammonium hydroxide and having a pH less than 13.3.

14. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkaline electrolyte containing about 100 to 200' grams of sodium carbonate per liter, and increasing the initial voltage of about 5 volts to 45 or more volts as the current density decreases.- EDWARD CUSHMAN TRUESDALE. ERNEST JOHN WILHELM. 

